LONDON, Sept 28 (Reuters) - Britain’s opposition Labour Party said a trade dispute between U.S. planemaker Boeing and its Canadian rival Bombardier should be referred to the World Trade Organization.

Prime Minister Theresa May said that Boeing was undermining its relationship with Britain by its behaviour in a dispute with Canadian rival Bombardier that has put 4,200 jobs at risk in Northern Ireland.

“Instead of putting ourselves in the wrong by threatening to victimise Boeing - an empty threat given that thousands of jobs in the UK rely on the aerospace sector - the UK as one of the founders of the World Trade Organization, should refer the case to the WTO dispute settlement procedure,” said Barry Gardiner, Labour’s international trade policy chief.

Britain is already involved in the world’s largest trade dispute, involving mutual claims of illegal aircraft subsidies between Europe and the United States. The case has led to years of argument over development loans given to aircraft manufacturers such as Bombardier. (Reporting by William James; editing by Guy Faulconbridge/Tim Hepher)